1. Field of the Invention
The present invention relates generally to hot melt inks, and more particularly to pigmented semiconductive hot melt inks and ink jet apparatus employing same.
2. Statement of the Prior Art
"Hot melt" inks (i.e., inks which are typically in a solid form at room temperature, but which are in a liquid form at a temperature that is above room temperature), and ink jet apparatus employing hot melt inks are well known. For example, Merritt et al. in U.S. Pat. No. 4,390,369 and U.S. Pat. No. 4,484,948 disclose various types of natural wax-containing, hot melt inks which are suitably employed in ink jet apparatus of the type disclosed by Howkins in U.S. Pat. No. 4,459,601. Each of the above noted patents is assigned to the assignee of the present invention and is incorporated herein by reference.
Pigmented inks are also well known. For example, it is known to employ carbon black as black pigment in inks. See, e.g., U.S. Pat. No. 3,660,133--Van Der Schuyt et al. Inks which contain pigments for their coloration are often preferred over dye-colored inks because they shrink less. However, pigmented inks are also often troublesome because of the requirement for constant pigment dispersion in order to maintain overall ink quality.
Carbon blacks (e.g., channel black, soft black, and furnace black, etc.) are crystallographically related to graphite and, as such, can be intrinsic semiconductors. Such carbon blacks, for example, have been used extensively in the plastics industry to render normally resistive polymers electrically conductive. Carbon blacks have also been used together with resin binders to produce "electrical resistor inks", which are inks that are used as direct substitutes for discrete resistors in all types of electrical circuits. See, e.g., U.S. Pat. No. 3,992,212--Youtsey et al. and U.S. Pat. No. 4,479,890--Prabhu et al. Those known electrical resistor inks, however, are of a liquid form. Such inks containing carbon black have been used in a variety of ink jet printing apparatus. For example, Nguyen et al. in U.S. Pat. No. 4,530,961 disclose low viscosity stable aqueous dispersions of graft carbon black in liquid inks suitable for use in "jet-printing" (i.e., continuous) machines.
Sachdev et al. in U.S. Pat. No. 4,549,824 also disclose the use of carbon black-containing inks in thermal ink transfer printing processes, and Fujimura et al. in U.S. Pat. No. 4,737,803 disclose the use of such inks in a thermal electrostatic ink-jet recording apparatus.
The advantages of hot melt inks and ink jet apparatus employing same when compared to liquid inks are legion and notoriously well known. However, ink jet apparatus which typically employs hot melt ink requires additional systems to melt the hot melt ink from its solid form to its liquid form, to maintain such liquid form in a stable state, and to supply that hot melt ink in such a stable, liquid form at the print head for ejection therefrom. Conventional hot melt ink jet printing apparatus utilize thermistors or other similar such heating means to accomplish these ends. For example, see U.S. Pat. No. 4,607,266--DeBonte and U.S. Pat. No. 4,791,439--Guiles, each of which is assigned to the assignee of the present invention and is incorporated herein by reference.
The two patents referred to immediately herein above do not disclose the use of hot melt inks containing carbon black. However, Berry et al. in U.S. Pat. No. 3,653,932 and Kurz et al. in U.S. Pat. No. 3,715,219 both disclose electrostatically-deflected ink jet apparatus which use hot melt inks containing carbon black for the purposes of achieving resistivities in the range of 10.sup.6 to 10.sup.11 ohm-centimeters. It is known that such resistivities are needed to achieve proper deflection of ink streams in electrostatic ink jet apparatus. Moreover, the specific apparatus that is disclosed in these patents includes heating means to maintain their inks in a liquid phase to the end that continuous flow thereof may be maintained to the nozzle.